Question

A "principal square root" is always...

Answer 1

Really none of the above.  

A principal square root of a real number is a positive real number (or a positive real number times i, if you're up to complex numbers, but let's assume not.)  It's not necessarily a positive integer, √2 being the obvious example.

Is √2 a positive fraction?   We can write it as √2 / 1 so I suppose it is.  It's certainly not a rational number.  Even if we grant irrationals as fractions surely √0=0 isn't a positive fraction.  So "always" isn't correct.

I'd go with none of the above, but if I had to choose I'd choose D.